1. Field of the Invention
The present invention relates to a display technique that offers, for example, a forgery-prevention effect.
2. Description of the Related Art
Generally, in order to prevent forgery of securities such as vouchers and checks, cards such as credit cards, cash cards, and ID cards, and certificates such as passports and driver's licenses, theses articles are provided with a display that offers a visual effect different from that offered by a common printed matter. In recent years, for other articles in addition to the above-described articles, distribution of forged articles has become an issue of social concern. Thus, opportunities to apply similar forgery-prevention technique to such articles are increasing.
A display including an arrangement of grooves as a diffraction grating is known as one of displays offering a visual effect different from that offered by a common printed matter. The display can be formed to display, for example, an image that changes in accordance with observation conditions or a stereoscopic image. Iridescent spectral colors displayed by the diffraction grating cannot be created by common printing techniques. Thus, a display including a diffraction grating is widely used for articles requiring forgery-prevention measures.
Jpn. Pat. Appln. KOKAI Publication No. 2-72320 describes arranging diffraction gratings different from each other in the lengthwise directions of the grooves or grating constants, i.e., pitches of grooves, so as to display a pattern. When the position of an observer or light source relative to the diffraction grating changes, the wavelength of diffracted light reaching eyes of the observer changes. Therefore, if the above configuration is adopted, an image changing iridescently can be displayed.
A display using a diffraction grating generally uses a relief-type diffraction grating. A relief-type diffraction grating is normally obtained by duplicating a pattern of a master produced using photolithography.
U.S. Pat. No. 5,058,992 describes a method of manufacturing a master having a relief-type diffraction grating thereon in which a plate-shaped substrate having a photosensitive resist applied thereon is placed on an XY stage, and the photosensitive resist is irradiated with an electron beam while moving the stage under the control of a computer so as to perform pattern exposure on the photosensitive resist. A master having a diffraction grating can also be formed by using two-beam interference.
In the production of a relief-type diffraction grating, first, a master is normally formed by one of methods as described above and then a metallic stamper is produced by, for example, an electroforming technique using the master. Next, the metallic stamper is used as a mother die to duplicate the relief-type diffraction grating. That is, first, a thermoplastic resin or photo-curable resin is applied to a film- or sheet-shaped thin transparent substrate made of, for example, polyethylene terephthalate (PET) or polycarbonate (PC). Next, a metallic stamper is brought into close contact with the coated film and heat or light is applied to the resin layer in this state. After curing the resin, the metallic stamper is removed from the cured resin so as to obtain a duplicate relief-type diffraction grating.
Generally, the relief-type diffraction grating is transparent. Thus, a reflection layer is normally formed on the resin provided with the relief structure by depositing a single layer or a plurality of layers of a metal such as aluminum or a dielectric by using the evaporation method.
Subsequently, the display obtained as described above is pasted on a substrate made of, for example, paper or a plastic film via an adhesive layer or sticky layer. Thus, the display adopting forgery-prevention measures is obtained.
The master used for producing the display including the relief-type diffraction grating is difficult to manufacture. Moreover, a relief structure needs to be transferred from the metallic stamper to the resin layer with high precision. That is, a high level of technique is required to produce a display including a relief-type diffraction grating.
However, as a result of the fact that the display including a relief-type diffraction grating is increasingly used in many articles requiring forgery-prevention measures, the technique is now widely recognized and accordingly, forgeries tend to increase. Thus, it is becoming increasingly more difficult to achieve a sufficient forgery-prevention effect by using a display only featured in that iridescent light is presented by diffracted light.